This invention relates to R.F. loop intrusion detection systems and in particular to means for improving the uniformity of detection sensitivity around the signal of radiating loop of such a system.
The R.F. Loop intruder Detection System consists of a loop of leaky coaxial cable which encircles the resource to be protected and acts as a distributed transmitting (or receiving) antenna. One or more monopole antennas located near the center of the loop act as receiving (or transmitting) elements. When an intruder crosses the leaky coax cable, he modifies the quiescent field conditions thereby producing a change in the received signal strength which is detected.
In the type of system to which the invention applies, the transmitter is connected to one end of the leaky coax cable and a matched load at the other. To protect a large aircraft, up to one thousand feet of leaky coaxial cable are required. However, because of the attenuation of the leaky coakial cable, the signal radiated from the far end of the cable is considerably less than that near the input end. Differences of ten decibles are typical. As a result, the detection sensitivity near the input end of the leaky cable sensor is greater than that near the termination end. This results in intrusion events at the far end of the loop going undetected. Increasing the sensitivity of the loop to ensure detection of intrusion at the far end has the effect of making the near end too sensitive resulting in false and nuisance alarms. Accordingly, there currently exists the need for means whereby R.F. loop intrusion detection systems can be made to exhibit adequate and substantially uniform detection sensitivity along the full length of the leaky transmission line loop. The present invention is directed toward meeting that need.